The invention relates to a process for the preparation of a Fischer-Tropsch catalyst which contains cobalt, zirconium or titanium, and silica. It relates further to the use of such a catalyst in the preparation of hydrocarbons by reaction of carbon monoxide with hydrogen.
The preparation of hydrocarbons from a H.sub.2 /CO mixture by contacting this mixture at elevated temperature and pressure with a catalyst is known in the literature as Fischer-Tropsch hydrocarbon synthesis. Catalysts often used for this purpose contain iron or cobalt together with one or more promoters and/or a carrier material. The conventional techniques of preparing Fischer-Tropsch catalysts are the precipitation route, the melting route and the impregnation route. Of these techniques the impregnation route is much to be preferred since this route is not so costly and/or time consuming, produces better reproducible results and generally yields materials with better catalytic properties. Briefly, the impregnation route involves contacting a porous carrier once or several times with an aqueous solution containing an iron or cobalt compound and, if desired, one or more compounds of the appropriate promoters, followed by removal of the liquid and calcination and reduction of the composition obtained.
The composition of the product obtained in the Fischer-Tropsch hydrocarbon synthesis by using catalysts prepared by impregnation is largely dependent on the catalytically active metal which is present on the catalyst. The use of a cobalt catalyst prepared by impregnation results in a product which consists mainly of unbranched paraffins. The use of an iron catalyst prepared by impregnation results in a product which, in addition to unbranched paraffins, contains a considerable quantity of olefins and oxygen-containing organic compounds. With a view to the composition of the product obtained, for the preparation of products suitable for use as motor fuels, preference is given to the use of a cobalt catalyst.
Earlier investigations have shown that catalysts prepared by impregnation of a silica carrier with an aqueous solution of a cobalt salt followed by calcination and reduction of the composition obtained showed excellent C.sub.3.sup.+ and C.sub.5.sup.+ selectivity but only relatively low activity. By including in the aqueous solution with which the silica was impregnated a zirconium or titanium salt in addition to the cobalt salt, the activity of the catalysts could be roughly doubled without loss of the afore-mentioned high C.sub.3.sup.+ and C.sub.5.sup.+ selectivity. A remarkable feature of these catalysts prepared by co-impregnation with the aid of an aqueous solution is the extremely narrow range of promoter load levels within which the promoter exercises its activity-promoting effect on the catalyst. It was found that, whereas for these catalysts the inclusion of a quantity of about 1 pbw of zirconium per 100 pbw of silica led to, roughly, doubling of their activity, the inclusion of larger quantities of zirconium caused a rapid decrease of activity promotion and, upon inclusion of a quantity of about 6 pbw per 100 pbw of silica, the activity-promoting affect was seen to have disappeared altogether. In the latter case a catalyst was obtained which had an activity almost equal to that of a catalyst containing nothing but cobalt.
Although the catalysts prepared by co-impregnation with the aid of an aqueous solution as described hereinbefore have acceptable activity, there is an unmistakable need for catalysts of this type having higher activity.
Continued investigation into this subject was carried out to determine whether changes in the technique of impregnation might lead to widening of the range within which the promoter exercises its activity-promoting effect on the catalyst. For this purpose replacement of the co-impregnation by separate impregnation was investigated in the first place. Catalysts were prepared by impregnating a silica carrier first with an aqueous solution of a cobalt compound and subsequently, after calcination of the cobalt-containing composition, impregnating the latter with an aqueous solution of a zirconium compound, followed by calcination and reduction of the zirconium and cobalt-containing composition. Thus were prepared catalysts containing different quantities of zirconium. It was found that just as in the case of the catalysts prepared by co-impregnation, so in the case of the catalysts prepared by separate impregnation did the incorporation of a quantity of about 1 pbw zirconium per 100 pbw of silica result in--roughly--doubled activity. In the catalysts prepared by separate impregnation the range within which the promoter exercises its activity-promoting effect on the catalyst was found to have decreased in comparison with the catalysts prepared by co-impregnation. Whereas in the catalysts prepared by co-impregnation the incorporation of about 6 pbw of zirconium per 100 pbw of silica resulted in an activity level corresponding to that of a catalyst containing nothing but cobalt, the catalysts prepared by separate impregnation containing about 6 pbw of zirconium per 100 pbw of silica were seen to be entirely inactive.
On the assumption that in the above-mentioned separate impregnation the use of an organic promoter compound dissolved in an organic solvent might lead to the purpose intended, catalysts were prepared by impregnating a silica carrier first with an aqueous solution of a cobalt compound and, after calcination of the cobalt-containing composition, impregnating the latter with a solution of an organic zirconium compound in an organic solvent, followed by calcination and reduction of the zirconium and cobalt-containing composition. Thus were prepared catalysts containing varying proportions of zirconium. It was found than when these catalysts were used for the preparation of hydrocarbons from H.sub.2 /CO mixtures, they behaved in precisely the same manner as the aforementioned catalysts which had been prepared by separate impregnation using an aqueous solution of an inorganic zirconium compound, viz. catalysts containing about 1 pbw zirconium per 100 pbw silica showed roughly doubled activity, whereas catalysts containing about 6 pbw zirconium per 100 pbw silica proved to be entirely inactive.
The above-mentioned disappointing results as to the effect of a change in the impregnation method upon the activity of the promoted Co/SiO.sub.2 catalysts notwithstanding, yet another attempt was made at achieving catalysts with improved activity by using the above-described separate impregnation, but with the order of the impregnation steps being reversed. This surprisingly led to the find that in this way it is possible to prepare promoted Co/SiO.sub.2 catalyts whose activity is 3-4 times as high as that of a catalyst containing nothing but cobalt. In order to achieve an improvement in activity which exceeds that which can be achieved by co-impregnation of the promoter together with the cobalt or the separate impregnation of the promoter after the cobalt, the proportion of promoter deposited on the silica prior to the cobalt should be at least 2 pbw per 100 pbw silica for catalysts containing 5-40 pbw cobalt per 100 pbw silica. Contrary to what is seen for the catalyts in which the promoter has been deposited on the carrier by co-impregnation together with the cobalt or by separate impregnation of the promoter after the cobalt, the range of promoter loads within which the promoter exercises its activity-promoting effect is very wide in the case of the catalysts in which the promoter is deposited on the carrier previous to the cobalt. In principle, when preparing these catalysts promoter quantities of up to a maximum of 150 pbw per 100 pbw silica may be considered.